Vlll PREFACE. 



obscure materials presented by physiological and pathological histo- 

 logy must necessarily be subjected to a critical examination before 

 they can be incorporated with physiological chemistry, and hence 

 the author regards such a course of self-training as indispensable 

 in the attempt to furnish his readers with a systematic arrange- 

 ment of facts. Moreover, those departments of science which 

 must serve as a basis to physiological chemistry, have been encum- 

 bered with an accumulated mass of observations, from which have 

 arisen numerous hypotheses successively displaced by others not 

 un frequently of an opposite character. We must, therefore, as far 

 as is possible, attempt to judge for ourselves if we would not be 

 continually drawn aside by the opinions which are ever rising and 

 falling amid the fluctuations of ephemeral literature. 



But the most important reason for the delay that has occurred 

 in the publication of the second volume is, that in Physiological 

 Chemistry, even more than in Zoo-Chemistry, we are obliged to de- 

 part from the sure ground of exact enquiry, and to proceed to the con- 

 sideration of chemico-vital processes, which lie beyond the scope of 

 direct observation, and are thus called upon to admit the correct- 

 ness of deductions, whose logical authority is not always easy of 

 recognition. Modern science has directed its highest energies to 

 this point of physiologico- chemical investigation : and it was there- 

 fore to be expected that this yet imperfectly cultivated soil would 

 give birth to a number of more or less ingenious hypotheses, which 

 can only be sifted by independent examination and positive inves- 

 tigations. But since even this protracted delay and the frequent 

 reconsideration of all the materials at his command, do not give as 

 satisfactory a result as the author could wish, he has at length 

 determined to send forth this attempt at a History of Physiological 

 Chemistry, trusting to the indulgence of those who are labouring 

 in the same cause. 



LEIPSIC, 



September, 1849. 



